The goal of nanomanufacturing is to develop practical methods for leveraging novel properties inherent in nanomaterials to yield viable new technologies. Realizing a practical and scalable platform for nanomanufacturing macroscopic materials and architectures via bottom-up self or directed assembly is a critical aspect of current nanotechnology research. In particular, reprogrammable mechanisms for assembling and then transferring large area nanomaterial assemblies into application platforms are needed, where the “raw” material is a nanoparticle or similar nanoscale material, and the finished product is a designed, macroscopic structure built from the raw materials. One approach to nanomaterials assembly uses external electromagnetic forces to position and assemble arrays of nanoparticles. Here, magnetic-gradient force directed assembly was originally employed by Bitter to visualize domains and surface features in magnetic materials. This “Bitter technique” has been applied for imaging the patterns created by magnetic recording on disk drive media, and for separating weakly magnetic and nonmagnetic particles through high gradient magnetic separation (HGMS). More recently, lithographed arrays of magnetic materials biased with external magnetic fields have been used to assemble magnetic particles in a fluidic environment, yielding techniques for particle transport for drug delivery, masking for lithography, and stitched membrane assembly.
Hard disk drives using magnetic recording technology store much of the world's information on Cobalt alloy magnetic media with grain sizes less than 10 nm in diameter, bit lengths in the 30 nm range, and track widths less than 100 nm, i.e. at significantly smaller length scales than the lithographed structures described above. Driven by explosive growth in the market for storage capacity over the past decade, the disk drive industry has leveraged giant magnetoresistance (GMR), advanced materials, and scalability to drive the core technologies to nanoscale dimensions, and in terms of required critical dimensions for lithography, the data storage roadmap closely matches the semiconductor industry. Presently, the industry is aiming toward a 1 Terabit/in2 areal density target where bit lengths and track widths will be 11 nm and 38 nm, respectively.
Enormous magnetic field gradients exist at recorded magnetization transitions in disk drive media, suggesting the application of magnetically recorded transitions for local magnetophoresis. From the conventional expressions describing the magnetic field of recorded transitions in disk media, it is found that the magnetic field gradient ranges from>4×106 T/m at 25 nm to˜5000 T/m at 1 μm above the surface. These field gradients are significantly higher than the≤1000 T/m gradients typically employed in external field-driven magnetophoresis. At 50 nm above the disk surface, the work done by this field gradient to move a 10 nm diameter nanoparticle (partially magnetized by the medium field at this height) one diameter toward the surface exceeds kBT, meaning that the magnetic gradient force will dominate over other transport mechanisms at this height for approximately 10 nm and larger nanoparticles. Therefore, these gradients allow for precision assembly of ferrite nanoparticles into large area patterns, and we have previously employed AFM to determine a coating-to-coating repeatability for this process of 27±11 nm.
Diffraction gratings consisting of a large number of equally spaced parallel slits or grooves play an important role in many technologies, including spectroscopy, laser systems, and information communication, where, for example, gratings increase the capacity of fiber-optic networks using wavelength divisionmultiplexing/demultiplexing. High-resolution commercial diffraction gratings were originally fabricated with ruling engines, and the ruling process is slow and requires precise control of mechanical motion and external vibration. Other fabrication methods include photographic recording of a stationary interference fringe field in photoresist to create a holographic grating, electron beam lithography, and focused ion beam etching. Recently, gratings have been fabricated using laser pulses to ablate metal nanoparticles or thin films, with interference to create the grating pattern. Given the evolving need for control over optical element fabrication, lower cost and sustainable manufacturing technologies with nanometer precision are needed to create novel optical materials, and maintain the pace of technological innovation in optical technologies.
Nanoparticle self-assembly has promise as a sustainable manufacturing technology for construction of complex patterns including linear chains, and close packed arrays. For optical applications, self-assembly has been used to create dynamic diffraction gratings in liquid from colloidal nanoparticles using electrophoresis. Similarly, self-assembly via DNA and other surface anchoring techniques has been employed to pattern diffraction gratings on surfaces.